EP0446552B1 - Superconductive compounds and process for producing said compounds - Google Patents
Superconductive compounds and process for producing said compounds Download PDFInfo
- Publication number
- EP0446552B1 EP0446552B1 EP90500017A EP90500017A EP0446552B1 EP 0446552 B1 EP0446552 B1 EP 0446552B1 EP 90500017 A EP90500017 A EP 90500017A EP 90500017 A EP90500017 A EP 90500017A EP 0446552 B1 EP0446552 B1 EP 0446552B1
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- EP
- European Patent Office
- Prior art keywords
- pellets
- compounds
- temperature
- mixture
- hours
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
- 150000001875 compounds Chemical class 0.000 title claims description 25
- 238000000034 method Methods 0.000 title claims description 14
- 239000008188 pellet Substances 0.000 claims description 19
- 239000000843 powder Substances 0.000 claims description 14
- 239000000203 mixture Substances 0.000 claims description 13
- 230000007704 transition Effects 0.000 claims description 9
- FGIUAXJPYTZDNR-UHFFFAOYSA-N potassium nitrate Chemical compound [K+].[O-][N+]([O-])=O FGIUAXJPYTZDNR-UHFFFAOYSA-N 0.000 claims description 8
- QPLDLSVMHZLSFG-UHFFFAOYSA-N Copper oxide Chemical compound [Cu]=O QPLDLSVMHZLSFG-UHFFFAOYSA-N 0.000 claims description 7
- 238000010438 heat treatment Methods 0.000 claims description 7
- 239000000919 ceramic Substances 0.000 claims description 4
- 238000000227 grinding Methods 0.000 claims description 4
- 239000011572 manganese Substances 0.000 claims description 4
- 229910044991 metal oxide Inorganic materials 0.000 claims description 4
- 150000004706 metal oxides Chemical class 0.000 claims description 4
- 238000002156 mixing Methods 0.000 claims description 4
- 229910000018 strontium carbonate Inorganic materials 0.000 claims description 4
- 229910052791 calcium Inorganic materials 0.000 claims description 3
- 239000011575 calcium Substances 0.000 claims description 3
- 239000010949 copper Substances 0.000 claims description 3
- GEYXPJBPASPPLI-UHFFFAOYSA-N manganese(III) oxide Inorganic materials O=[Mn]O[Mn]=O GEYXPJBPASPPLI-UHFFFAOYSA-N 0.000 claims description 3
- 238000004519 manufacturing process Methods 0.000 claims description 3
- 235000010333 potassium nitrate Nutrition 0.000 claims description 3
- 239000004323 potassium nitrate Substances 0.000 claims description 3
- 238000003825 pressing Methods 0.000 claims description 3
- LEDMRZGFZIAGGB-UHFFFAOYSA-L strontium carbonate Chemical compound [Sr+2].[O-]C([O-])=O LEDMRZGFZIAGGB-UHFFFAOYSA-L 0.000 claims description 3
- OYPRJOBELJOOCE-UHFFFAOYSA-N Calcium Chemical compound [Ca] OYPRJOBELJOOCE-UHFFFAOYSA-N 0.000 claims description 2
- 229910052788 barium Inorganic materials 0.000 claims description 2
- 238000001354 calcination Methods 0.000 claims description 2
- 229910052802 copper Inorganic materials 0.000 claims description 2
- 229910052760 oxygen Inorganic materials 0.000 claims description 2
- 229910052712 strontium Inorganic materials 0.000 claims description 2
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 claims 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims 1
- DSAJWYNOEDNPEQ-UHFFFAOYSA-N barium atom Chemical compound [Ba] DSAJWYNOEDNPEQ-UHFFFAOYSA-N 0.000 claims 1
- WPBNNNQJVZRUHP-UHFFFAOYSA-L manganese(2+);methyl n-[[2-(methoxycarbonylcarbamothioylamino)phenyl]carbamothioyl]carbamate;n-[2-(sulfidocarbothioylamino)ethyl]carbamodithioate Chemical compound [Mn+2].[S-]C(=S)NCCNC([S-])=S.COC(=O)NC(=S)NC1=CC=CC=C1NC(=S)NC(=O)OC WPBNNNQJVZRUHP-UHFFFAOYSA-L 0.000 claims 1
- 239000001301 oxygen Substances 0.000 claims 1
- CIOAGBVUUVVLOB-UHFFFAOYSA-N strontium atom Chemical compound [Sr] CIOAGBVUUVVLOB-UHFFFAOYSA-N 0.000 claims 1
- 239000000463 material Substances 0.000 description 9
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 4
- 239000002887 superconductor Substances 0.000 description 4
- 229910002480 Cu-O Inorganic materials 0.000 description 3
- 229910052748 manganese Inorganic materials 0.000 description 3
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical group [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 2
- 230000006399 behavior Effects 0.000 description 2
- 229910052797 bismuth Inorganic materials 0.000 description 2
- JCXGWMGPZLAOME-UHFFFAOYSA-N bismuth atom Chemical compound [Bi] JCXGWMGPZLAOME-UHFFFAOYSA-N 0.000 description 2
- AMWRITDGCCNYAT-UHFFFAOYSA-L hydroxy(oxo)manganese;manganese Chemical compound [Mn].O[Mn]=O.O[Mn]=O AMWRITDGCCNYAT-UHFFFAOYSA-L 0.000 description 2
- 239000007788 liquid Substances 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 238000012986 modification Methods 0.000 description 2
- 230000004048 modification Effects 0.000 description 2
- 229910052757 nitrogen Inorganic materials 0.000 description 2
- 229910052761 rare earth metal Inorganic materials 0.000 description 2
- 239000000523 sample Substances 0.000 description 2
- 238000003746 solid phase reaction Methods 0.000 description 2
- 238000010671 solid-state reaction Methods 0.000 description 2
- 229910015901 Bi-Sr-Ca-Cu-O Inorganic materials 0.000 description 1
- 239000005751 Copper oxide Substances 0.000 description 1
- -1 R=Y Inorganic materials 0.000 description 1
- BQCADISMDOOEFD-UHFFFAOYSA-N Silver Chemical compound [Ag] BQCADISMDOOEFD-UHFFFAOYSA-N 0.000 description 1
- 229910009203 Y-Ba-Cu-O Inorganic materials 0.000 description 1
- 229910052784 alkaline earth metal Inorganic materials 0.000 description 1
- 150000001342 alkaline earth metals Chemical class 0.000 description 1
- 229910010293 ceramic material Inorganic materials 0.000 description 1
- 229910000431 copper oxide Inorganic materials 0.000 description 1
- 230000001351 cycling effect Effects 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 238000002050 diffraction method Methods 0.000 description 1
- BDAGIHXWWSANSR-NJFSPNSNSA-N hydroxyformaldehyde Chemical compound O[14CH]=O BDAGIHXWWSANSR-NJFSPNSNSA-N 0.000 description 1
- 230000003287 optical effect Effects 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 238000002360 preparation method Methods 0.000 description 1
- 150000002910 rare earth metals Chemical class 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 229910052709 silver Inorganic materials 0.000 description 1
- 239000004332 silver Substances 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
- VWQVUPCCIRVNHF-UHFFFAOYSA-N yttrium atom Chemical compound [Y] VWQVUPCCIRVNHF-UHFFFAOYSA-N 0.000 description 1
Images
Classifications
-
- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
- C04B35/01—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
- C04B35/45—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on copper oxide or solid solutions thereof with other oxides
-
- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N60/00—Superconducting devices
- H10N60/80—Constructional details
- H10N60/85—Superconducting active materials
- H10N60/855—Ceramic superconductors
- H10N60/857—Ceramic superconductors comprising copper oxide
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10S505/00—Superconductor technology: apparatus, material, process
- Y10S505/725—Process of making or treating high tc, above 30 k, superconducting shaped material, article, or device
- Y10S505/739—Molding, coating, shaping, or casting of superconducting material
Definitions
- the present invention relates to superconductive compounds which show superconductive characteristics in multiple phases at temperatures ranging from approximately 87 K to 310 K, and to a process for the production of said compounds.
- the present invention provides superconductive compounds with new compositions, based on metal oxides, which present higher superconducting transition temperatures, as well as a process for obtaining the same.
- the present invention also provides a process for producing such compounds, which process consists essentially of a solid state reaction with specific times, temperatures and oxidation conditions, which determine the characteristics of the compounds.
- our new compounds present optical characteristics different from the already well-known 90 K superconductors.
- a primary object of this invention is to provide new compositions of manganese-based compounds which consist of Mn, Cu, O, and one or more alkaline earth metal selected from the group consisting of Ca, Ba and Sr, and which present superconducting transition temperatures in the range of from 87 K to 310 K, approximately.
- Another object is to provide a process for producing these new compounds, specifying the parameters which insure the reproducibility of the results.
- the compounds are obtained by mixing powders of manganese oxide, strontium carbonate and copper oxide.
- the process for producing the above-mentioned compounds comprises first mixing appropriate amounts of the original components and then pressing the mixture to form pellets.
- said process comprises :
- the pressed powders ure subjected to the following heat treatment Temperature °C Time (hours) 600 14 800 22 850 25
- the pellets are ground to a fine powder and reheated for 27 hours at 800°C.
- the process is repeated at 850°C, for the same period of time.
- the materials are reground and reheated to 900°C. for 12 hours. The last step is repeated once more.
- Potassium nitrate is then added to the powder mixture and mixed thoroughly.
- the powders are pressed into pellets and reheated at 900°C, for 12 hours.
- Figure 1 shows a resistance vs. temperature plot of a typical manganese-based sample of the present invention, showing a resistance drop temperature at 310 K, and other resistance drop to zero at a temperature of 87 K.
- powders of Mn 2 O 3 , SrCO 3 and CuO of high purity are mixed, for example, in the proportion 2:2:3.
- a pressed powder pellet of 1 cm in diameter and 0.5 cm thick is prepared with a pressure of 10 Kg/cm 2 .
- the resulting pellet is heated to a temperature of 600°C for approximately 14 hours.
- the temperature is then raised to a value of 800°C, and maintained approximately 22 hours.
- the temperature of the pellet is increased to 850°C and maintained for approximately 25 hours.
- the compound obtained is a solid compact, blackish (bluish black) pellet.
- the pellet is ground and calcinated in the following sequence: Temperature °C Time (hours) 800 27 (reground) 850 27 (reground) 900 12 (reground) 900 12 (reground)
- KNO 3 is added to the powder mixture in an amount of 10% by weight of the powders, and the mixture is ground until homogeneity is reached.
- the powder is pressed into pellets which are heated at 900°C for approximately 12 hours.
- Figure 1 shows a typical plot of resistance vs. temperature of the compound prepared in this way.
- the superconducting state is characterized by zero resistance measurement within the experimental precision.
- the resistance measurements were made by the usual fourpoint-probe technique in different configurations using silver point contacts.
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- Engineering & Computer Science (AREA)
- Chemical & Material Sciences (AREA)
- Ceramic Engineering (AREA)
- Manufacturing & Machinery (AREA)
- Materials Engineering (AREA)
- Structural Engineering (AREA)
- Organic Chemistry (AREA)
- Compositions Of Oxide Ceramics (AREA)
- Inorganic Compounds Of Heavy Metals (AREA)
Description
- The present invention relates to superconductive compounds which show superconductive characteristics in multiple phases at temperatures ranging from approximately 87 K to 310 K, and to a process for the production of said compounds.
- The recent discovery of high transition temperature superconductors represents a possible technological revolution and has attracted the interest of scientists from around the world. These new materials are ceramics, made from metal oxides.
- During the past two years, many studies have been reported following the discovery of the efficacy of La-Ba-Cu-O materials. This system has been extensively analyzed and has a transition temperature around 30 K. Subsequently a Y-Ba-Cu-O material having a 90 K transition temperature was obtained. Many other 90 K superconducting materials have been obtained substituting a rare earth element for the yttrium.
- From X-Ray studies the crystallography of these 90 K compounds is well known. These superconductors (R-Ba-Cu-O, R=Y, rare earth) have been called 1,2,3 compounds because of their stoichiometry and present an orthorhombic unit cell, which can be visualized as a stacking of three perovskites.
- Compounds based on bismuth (Bi-Sr-Cu-O) which show superconductive characteristics with transition temperatures in the range from 90 K to 120 K have also been reported. The bismuth-based compounds have been modified by adding calcium, producing superconductive ceramic compounds of the type Bi-Sr-Ca-Cu-O.
- From the point of view of possible applications of these materials, it is important to emphasize that they are superconductors above the liquid nitrogen temperature. However, an increase in their transition temperature widens technological possibilities even more.
- As it can be inferred from the above, the technology developed so far is greatly limited by the fact that, in order to achieve the phenomenon of superconductivity, it is necessary to cool the materials by means of liquid nitrogen, which represents a serious limitation as to the possibility of using such materials for practical applications. Hence, ongoing research has focused on the development of superconductive materials which can operate efficiently at higher temperatures, ideally at ambient temperatures.
- All of these ceramic materials are produced through solid state reactions of appropriate amounts of the original oxides. Nevertheless, what determines the quality and characteristics of the products , other than their composition, are the times, temperatures and conditions involved in each step of the process, as well as the number of times the steps are repeated.
- The present invention provides superconductive compounds with new compositions, based on metal oxides, which present higher superconducting transition temperatures, as well as a process for obtaining the same. The present invention also provides a process for producing such compounds, which process consists essentially of a solid state reaction with specific times, temperatures and oxidation conditions, which determine the characteristics of the compounds. In addition, our new compounds present optical characteristics different from the already well-known 90 K superconductors.
- A primary object of this invention is to provide new compositions of manganese-based compounds which consist of Mn, Cu, O, and one or more alkaline earth metal selected from the group consisting of Ca, Ba and Sr, and which present superconducting transition temperatures in the range of from 87 K to 310 K, approximately.
- Another object is to provide a process for producing these new compounds, specifying the parameters which insure the reproducibility of the results.
- The compounds are obtained by mixing powders of manganese oxide, strontium carbonate and copper oxide.
- It has been found that the use of these particular components results in compounds which present superconducting transition temperatures in the range from 87 K to 310 K, approximately. It is important to note that in these new stoichiometries the type of basic components is different from those known in the art.
- The process for producing the above-mentioned compounds comprises first mixing appropriate amounts of the original components and then pressing the mixture to form pellets.
- More specifically, said process comprises :
- a) mixing powders of Mn2O3, SrCO3, and CuO ;
- b) subjecting said mixture to a pressure of at least 10kg/cm2, to form powder pellets ;
- c) heating the pellets by raising the temperature stepwise from 600 to 850°C, each temperature rise occuring at time intervals of 10 to 30 hours ;
- d) grinding and calcinating the pellets, through repetition of heating and regrinding ;
- e) adding potassium nitrate to the ground pellets, and grinding the mixture ;
- f) pressing the mixture into pellets and heating at 900°C for about 12 hours.
- In one example, the pressed powders ure subjected to the following heat treatment:
Temperature °C Time (hours) 600 14 800 22 850 25 - Thereafter, the pellets are ground to a fine powder and reheated for 27 hours at 800°C. The process is repeated at 850°C, for the same period of time. Afterwards, the materials are reground and reheated to 900°C. for 12 hours. The last step is repeated once more.
- Potassium nitrate is then added to the powder mixture and mixed thoroughly.
- Finally, the powders are pressed into pellets and reheated at 900°C, for 12 hours.
- Figure 1 shows a resistance vs. temperature plot of a typical manganese-based sample of the present invention, showing a resistance drop temperature at 310 K, and other resistance drop to zero at a temperature of 87 K.
- As mentioned in the above summary, powders of Mn2O3, SrCO3 and CuO of high purity are mixed, for example, in the proportion 2:2:3.
- They are ground until homogeneity is reached and a grain size of several microns is obtained. A pressed powder pellet of 1 cm in diameter and 0.5 cm thick is prepared with a pressure of 10 Kg/cm2. The resulting pellet is heated to a temperature of 600°C for approximately 14 hours. The temperature is then raised to a value of 800°C, and maintained approximately 22 hours. The temperature of the pellet is increased to 850°C and maintained for approximately 25 hours. The compound obtained is a solid compact, blackish (bluish black) pellet.
- The pellet is ground and calcinated in the following sequence:
Temperature °C Time (hours) 800 27 (reground) 850 27 (reground) 900 12 (reground) 900 12 (reground) - Thereafter, KNO3 is added to the powder mixture in an amount of 10% by weight of the powders, and the mixture is ground until homogeneity is reached. The powder is pressed into pellets which are heated at 900°C for approximately 12 hours.
- The variation of the resistance vs. temperature of the compound is measured. Figure 1 shows a typical plot of resistance vs. temperature of the compound prepared in this way.
- Cycling the temperature of the compound between ambient temperature and 4.2K proves that these behaviors are not affected by the temperature changes. Preparation of the samples made following this process was repeated several times, showing the same behavior.
- As it is well known, the superconducting state is characterized by zero resistance measurement within the experimental precision. In this case, the resistance measurements were made by the usual fourpoint-probe technique in different configurations using silver point contacts.
- While the principles and general procedures of the invention have been described according to one preferred embodiment, it will immediately be obvious to those who are skilled in the art that many changes and modifications may be made, without departing from the scope of this invention as set forth in the appended claims.
- The appended claims are therefore intended to cover and embrace such modifications.
Claims (3)
- A process for preparing a ceramic superconductive compound, based on metal oxides, characterized in that one prepares a superconductive compound consisting of manganese, copper, oxygen, and of one or more elements (A) selected from the group of calcium, barium and strontium, wherein the ratio Mn:(A):Cu, is of 4:2:3, and having a superconducting transition temperature range between about 87 K and 310 K.
- A process for preparing a ceramic superconductive compound, based on metal oxides according to claim 1, characterized in that said process comprises :a) mixing powders of Mn2O3, SrCO3, and CuO in the proportion 2:2:3;b) subjecting said mixture to a pressure of at least 10kg/cm2, to form powder pellets ;c) heating the pellets by raising the temperature stepwise from 600 to 850°C, each temperature rise occurring at time intervals of 10 to 30 hours ;d) grinding and calcinating the pellets, through repetition of heating and regrinding ;e) adding potassium nitrate to the ground pellets, and grinding the mixture ;f) pressing the mixture into pellets and heating at 900°C for about 12 hours.
Priority Applications (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
DE69029260T DE69029260D1 (en) | 1989-02-16 | 1990-03-16 | Superconducting compositions and processes for their manufacture |
EP90500017A EP0446552B1 (en) | 1989-02-16 | 1990-03-16 | Superconductive compounds and process for producing said compounds |
US08/392,403 US5583096A (en) | 1989-02-16 | 1995-02-22 | Superconductive compounds and process for producing said compounds |
Applications Claiming Priority (5)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US31106689A | 1989-02-16 | 1989-02-16 | |
EP90500017A EP0446552B1 (en) | 1989-02-16 | 1990-03-16 | Superconductive compounds and process for producing said compounds |
US71718091A | 1991-06-18 | 1991-06-18 | |
US82408392A | 1992-01-22 | 1992-01-22 | |
US11525193A | 1993-09-01 | 1993-09-01 |
Publications (2)
Publication Number | Publication Date |
---|---|
EP0446552A1 EP0446552A1 (en) | 1991-09-18 |
EP0446552B1 true EP0446552B1 (en) | 1996-11-27 |
Family
ID=40097428
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP90500017A Expired - Lifetime EP0446552B1 (en) | 1989-02-16 | 1990-03-16 | Superconductive compounds and process for producing said compounds |
Country Status (3)
Country | Link |
---|---|
US (1) | US5583096A (en) |
EP (1) | EP0446552B1 (en) |
DE (1) | DE69029260D1 (en) |
Families Citing this family (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US20030138372A1 (en) * | 1999-04-28 | 2003-07-24 | The Research Foundation Of State University Of New York | Method for identifying and synthesizing high dielectric constant perovskites |
Family Cites Families (6)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
FR2267290B1 (en) * | 1974-04-12 | 1976-12-17 | Anvar | |
FR2612507B1 (en) * | 1987-03-19 | 1989-05-05 | Comp Generale Electricite | COPPER OXIDE WITH SUPERCONDUCTING VALENCIA AND METHOD FOR IMPLEMENTING SAME |
US5032570A (en) * | 1987-08-04 | 1991-07-16 | Hitachi Metals, Ltd. | Method for producing ceramic superconducting material using intermediate products |
NZ226103A (en) * | 1987-09-11 | 1991-04-26 | Grace W R & Co | Preparation of superconducting mixed metal oxide ceramics with minimal extraneous matter |
FR2620865B1 (en) * | 1987-09-18 | 1990-04-20 | Rhone Poulenc Chimie | PROCESS FOR THE PREPARATION OF SUPERCONDUCTORS AND PRODUCTS THUS OBTAINED |
US5004725A (en) * | 1989-04-27 | 1991-04-02 | Ovonic Synthetic Materials Company Inc. | Parametrically modified superconductor material |
-
1990
- 1990-03-16 DE DE69029260T patent/DE69029260D1/en not_active Expired - Lifetime
- 1990-03-16 EP EP90500017A patent/EP0446552B1/en not_active Expired - Lifetime
-
1995
- 1995-02-22 US US08/392,403 patent/US5583096A/en not_active Expired - Fee Related
Also Published As
Publication number | Publication date |
---|---|
DE69029260D1 (en) | 1997-01-09 |
EP0446552A1 (en) | 1991-09-18 |
US5583096A (en) | 1996-12-10 |
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